This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 99 13435 filed in France on Oct. 27, 1999; the entire content of which is hereby incorporated by reference.
1. Filed of the Invention
The invention relates to a process for the purification of aqueous hydrogen peroxide solutions using ion-exchange resins. More particularly, the invention relates to a process for the production of aqueous hydrogen peroxide solutions containing very small amounts of impurities, especially metallic impurities, and intended for the fabrication of semiconductors, this production process being carried out very close to or at the point of use of the aqueous solution.
The invention also relates to a plant for producing aqueous hydrogen peroxide solutions containing very small amounts of impurities.
2. Description of the Related Art
Increasing the capacity of memories produced in the form of integrated circuits goes hand in hand with increasing purity of the chemicals used for the fabrication of chips on which these integrated circuits are produced.
Between 1985 and 1990, the capacity of on-chip memories was between 1 Mbits and 16 Mbits for an etching line thickness of between 1.5 xcexcm and 0.8 xcexcm, and required hydrogen peroxide solutions each with an impurity concentration that had to be less than 100 ppb.
At the present time, producing a 64 Mbit memory on a chip of the same size requires a line width of approximately 0.35 xcexcm and in general uses a grade of hydrogen peroxide having a maximum degree of impurity for each impurity lying within the range from 0.1 to 1 ppb.
Semiconductor manufacturers hope in the near future to be able to market 256 Mbit and 1 Gigabit memories with a minimum etching geometry of less than 0.18 xcexcm. The increase in memory capacity will then require a product for which the amount of each impurity will have to be less than 50 ppt.
Hydrogen peroxide is generally manufactured by auto-oxidation of an anthraquinone derivative or of a mixture of such derivatives. The said anthraquinone derivative(s) is (are) used dissolved in a complex mixture of organic solvents, such as an aromatic hydrocarbon mixed with an ester or an alcohol. This solution forms the working solution. This working solution is firstly hydrogenated in the presence of a catalyst, which converts the quinones into hydroquinones. It is then oxidized by bringing it into contact with air or with oxygen-enriched air. During this oxidation, the hydroquinones are oxidized, again, into quinones, with the simultaneous formation of hydrogen peroxide. The said hydrogen peroxide is extracted with water and the working solution undergoes a regeneration treatment before being used again.
The raw aqueous hydrogen peroxide solution is generally concentrated by rectification and purified in aluminium or stainless-steel distillation columns.
After this step, the aqueous hydrogen peroxide solution still contains impurities such as organic substances coming from the anthraquinone derivatives, solvents as well as degraded products from these compounds, and metallic elements such as aluminium, iron, chromium and zinc coming from the surface of the materials and pipes used. This hydrogen peroxide solution must therefore undergo a subsequent treatment in order to achieve the degree of purity required by the semiconductor industry.
Various techniques may be used to purify such a solution, such as distillation, crystallization, passage over beds of adsorbent resins and/or ion exchanges, reverse osmosis, filtration, ultrafiltration, etc.
In general, the organic substances are well purified by a distillation process and/or a process involving an adsorbent resin. For more details about these processes, reference may be made to Patents FR-A-2,710,045, EP-A-835,842, EP-A-502,466 and/or FR-A-1,539,843. The metallic elements, present in not insignificant amounts for applications in microelectronics, as well as anions such as nitrates or sulphates for example, are generally removed by passage over beds of ion-exchange resins.
Various methods of purifying aqueous hydrogen peroxide solutions using ion-exchange resins have been proposed in the literature. In general, these methods comprise making the solutions come into contact with at least one highly acid cation-exchange resin, obtained by polymerization of styrene and crosslinking by divinylbenzene followed by a sulphuric acid treatment, as well as at least one highly basic anion-exchange resin, obtained by the reaction of a tertiary amine, for example trimethylamine, with polychloro-methylstyrene.
In general, a person skilled in the art knows that the hydroxide form of the anionic resin is to be proscribed, since hydrogen peroxide very rapidly decomposes when in contact with it. The carbonate CO32xe2x88x92 and bicarbonate HCOxe2x88x923 forms of lower basicity may be used and are described, for example, in U.S. Pat. No. 3,294,488, U.S. Pat. No. 3,305,314 and/or U.S. Pat. No. 3,297,404.
However, the hydrogen peroxide decomposition reaction, when hydrogen peroxide comes into contact with this slightly basic support, is still possible, particularly if the hydrogen peroxide remains in static contact with the resin for several tens of minutes at room temperature.
Those skilled in the art know that this hydrogen peroxide decomposition reaction is accelerated by certain metals, such as iron and chromium, which may be contained in the resin itself and come from the materials used for its synthesis. For this purpose, special methods of preparing the resins before their use have been developed. Thus, JP-A-08,337,405 describes a process for treating anionic and cationic resins before use by an ultrapure aqueous solution of a mineral acid (for example, HCl). After this acid treatment, the resins are rinsed in ultrapure water. Next, the anionic resin is treated by an aqueous sodium hydroxide solution and then by a sodium carbonate or bicarbonate solution before being rinsed with ultrapure water. These treatments are generally lengthy and particular care has to be taken when carrying them out so as to avoid any contamination.
It is also known that hydrogen peroxide decomposition can gradually increase during purification of the said peroxide by the increase in metals picked up by the resin and carried away by the hydrogen peroxide solution itself. The addition of a mineral acid HX to the hydrogen peroxide solution before it comes into contact with the anionic resin, as described in U.S. Pat. No. 5,200,166, or with the cationic resin, as described in U.S. Pat. No. 5,534,238, makes it possible to reduce the evolution of oxygen due to hydrogen peroxide decomposition.
These methods have the drawback of considerably reducing the volume of hydrogen peroxide that can be purified per litre of resin. For example, in the case of anionic resins, sites are occupied by X, X being added in not insignificant amounts compared with the anionic mineral impurities contained in the hydrogen peroxide. This point is particularly important in the case of industrial plants intended for producing hydrogen peroxide. This is because, for the same volume of hydrogen peroxide, the amount of resin needed for purification will be greater if an HX resin is added to the hydrogen peroxide.
EP-A-846,654 discloses a process and an apparatus for purifying aqueous hydrogen peroxide solutions, in which the unpurified or partially purified solution flows under gravity through anionic and cationic resins. Because of the head losses due to the resins, the operating flow rate is limited because the flow is only created by gravity, thereby considerably reducing the production capacity. In this method of implementation, the cationic resin used is pretreated with an acid so as to remove the traces of metals.
Furthermore, the apparatus described does not benefit from a safety system allowing the unit to be rapidly shut down if the temperature in the latter rises abruptly, with a risk of explosion which must always be under control in hydrogen peroxide treatment units.
Thus, the on-site generation of extremely pure hydrogen peroxide remains a problem at the present time, even when prepurified hydrogen peroxide is used.
The subject of the present invention is a process for the additional purification of a hydrogen peroxide solution, especially of prepurified hydrogen peroxide, which does not have the disadvantages of the prior art and by virtue of which it is possible, without any danger, to produce an ultrapure-grade hydrogen peroxide intended for the semiconductor industry. According to a preferred version of the invention, the hydrogen peroxide solution is prepurified by distillation.
The invention relates more particularly to a process for the on-site purification of an aqueous hydrogen peroxide solution, in which the solution is made to pass through a resin bed capable, at least partially, of adsorbing or absorbing the impurities present in the solution, which process is characterized in that the hydrogen peroxide solution is injected into the resin bed and passes through the latter at an approximately linear velocity preferably of between 10 m/h and 50 m/h and more preferably between 10 m/h and 20 m/h and in that the resin bed is kept substantially compacted, preferably for at least 50% of the time during which the solution is being purified by coming into contact with the said resin.
According to the invention, preferably at least two columns are used, these being mounted in series and each containing an ion-exchange resin or an adsorbent resin (it is possible to have a single column, especially if the latter contains successive beds of anionic and/or cationic resins). Preferably, highly acid cationic resins, obtained by the sulphonation of a styrene-divinylbenzene copolymer, will be used. Also preferably, type-1 anionic resins will be used, these being obtained by the amination of a chloromethylstyrene-divinylbenzene copolymer, the ionic form of the resins being the carbonate or bicarbonate form. Advantageously, these resins are particularly oxidation-resistant.
The ion-exchange resins used will preferably consist of balls characterized by an approximately uniform diameter, thereby resulting in a coefficient of uniformity close to 1, the diameter of the said balls preferably being less than or equal to 700 xcexcm. The particle size of the resin balls according to this preferred method of implementing the invention allows higher rates of exchange reaction than with conventional resins and therefore produces hydrogen peroxide solutions of very high purity.
According to another aspect of the invention, resins comprising less than 50 mg of iron and/or less than 10 mg of copper and/or less than 50 mg of aluminium per litre of dry resin will be used: it has in fact been found that these small-diameter resins having low concentrations of metallic elements, such as those described above, allow a particularly pure hydrogen peroxide to be obtained. In particular, it is thus possible to dispense with the acid pretreatment needed with the resins of the prior art before they are introduced into the purification column.
According to a preferred method of implementing the invention, use is made of an anionic resin and/or a cationic resin, these being located in beds placed sequentially in the same column or in separate columns.
Preferably, the anionic resin will be placed in the first column (first anionic purification), the cationic resin optionally being placed in the second column).
Preferably, the liquid will flow upwards through the columns, and particularly through the column which contains the anionic resin, so as to promote the removal of gases such as, for example, carbon dioxide which comes from exchange of the anions dissolved in the hydrogen peroxide with the carbonate or bicarbonate ions of the resin.
Advantageously, the apparatus according to the invention comprises a system for distributing the liquid at the bottom of each of the columns (of the funnel type, for example) so as to allow even distribution of the liquid over the cross section of the column. The formation of preferred paths in the column, which reduce the effectiveness of the purification, is thus avoided.
Advantageously, the apparatus according to the invention comprises a device which retains the resin balls in each of the columns so that the resin bed is not lifted up by the ascending flow of liquid. This device may furthermore make it possible to keep the resin compacted in the column by exerting a slight pressure on the bed.
In addition, this device constitutes a safety device. This is because, in the event of an overpressure in the column, the device is pushed back towards the outside of the column, allowing the resin bed to expand and releasing the gases, such as the oxygen formed by hydrogen peroxide decomposition, more rapidly. Thus, when this device is of the type consisting of a sliding cover in the column, as described in the figures below, this sliding cover has the double function of, on the one hand, keeping pressure on the resin so as to keep it compacted, according to the invention, and, on the other hand, being able to slide upwards and be ejected from the column in the event of an overpressure in the latter arising from decomposition of the hydrogen peroxide, thus providing a safety function.
According to a preferred aspect of the invention, it has been found that excellent hydrogen peroxide purification is obtained if, on the one hand, the velocity of the liquid passing through the resin bed (preferably upwards) is approximately linear (that is to say the velocity vector essentially has a vertical component, of approximately constant modulus to within the head losses) and, on the other hand, the resin bed through which the hydrogen peroxide passes is kept substantially compacted over a significant part of at least one of the resin beds used. The expression xe2x80x9csubstantially compactedxe2x80x9d should be understood to mean that an additional force having a vertical component is exerted on each ball, in addition to the force resulting from the weight of the other balls lying above this ball. Preferably, this additional force may be applied for approximately the entire duration of implementation of the purification process.
According to another preferred characteristic, the liquid and/or gas in the columns is at a pressure close to atmospheric pressure. This is particularly important for the safety of the apparatus in the event of hydrogen peroxide decomposition; only the resin particles are exposed, at least partially, to a pressure greater than atmospheric pressure (at least partially compacted bed), while the liquid and/or gases are maintained at atmospheric pressure.
In order for the purification of hydrogen peroxide in solution to be carried out properly, the resin bed is kept substantially compacted for at least 50% of the time during which the solution is being purified by coming into contact with the said resin. This is because, when the resin particles are separated from each other (something which would generate a fluidized or partially fluidized resin bed), the efficiency of the purification greatly decreases, especially whenever less than approximately 50% of the purification takes place in contact with a compacted resin, that is to say a resin in which less than approximately 50% of the particles are in contact with each other (less than 50% of the purification taking place by contact with compacted resins may also mean, according to the invention, a resin bed for which, on average, over the entire height of the bed, less than 50% of the particles are in contact with each other, but also a succession of beds in which the first bed, for example, is in fluidized form and in which the second bed, for example, is in compacted form, or vice versa (in general, it may be considered that when at least some of the resin particles are substantially in contact with at least one other particle then the resin is substantially compacted).
Preferably, the resin bed is kept substantially compacted by applying pressure to the resin particles or balls forming the bed when the solution is injected upwards into the said bed in order to pass through it, at least partially.
The resin is, according to a preferred method, stored in a column and forms a bed of height h above which an approximately plane surface is placed at a distance d from the resin, the ratio d/h being kept under all circumstances less than approximately 0.1, preferably  less than 0.05 and more preferably  less than 0.01.
Preferably, a relative pressure of at least 100 pascals, preferably at least 200 pascals, will be applied to at least part of the (anionic and/or cationic) resin bed.
Preferably, the resin bed will consist of monospheres having diameters which are approximately identical to within 10%.
According to one version of the invention, in which the resin is placed in the form of a bed in one column, the solution is injected into the base of the said column using an injection nozzle which injects the liquid in a solid angle which substantially joins the walls of the column so as to inject the solution substantially over the entire cross section of the resin bed.
Preferably, the solution is injected into the resin bed via the lower part of the latter using a pumping system. It is also possible to provide several successive anionic and/or cationic resin beds.
According to one version, the apparatus according to the invention comprises several successive columns for purifying the solution, each having at least one anionic and/or cationic resin bed, the said beds being placed approximately at the same height.
Preferably, bubble traps are provided between the pumps and the point where the liquid is injected into the purification column.
According to one version of the invention, in order to provide greater safety, the temperature of the liquid in at least one resin bed is measured so as to detect any abnormal temperature rise which triggers the opening of a water circuit, causing the water to flood the resin bed. In one case, the water is generally injected into the base of the resin bed and rises up through the latter.
According to another version of the invention, nitrogen is injected above the resin bed. In general, the injected nitrogen will preferably have a so-called xe2x80x9celectronicxe2x80x9d purity adapted to the gas purity required in the fabrication of semiconductors, especially those which will be subsequently fabricated using the liquid solution. Preferably, the nitrogen will be injected into all the columns and the buffer tanks so as to prevent any contact between the ambient air and the liquid.
The liquid preferably flows from each column by overflow into a tank. The liquid is then pumped from this tank to the next column. When the tank is placed after the last column of the apparatus, the liquid may be pumped to the reject points or to a storage tank or to the top of the first column of the apparatus so as to circulate in a loop. Advantageously, the hydrogen peroxide solution circulates in a loop in order to allow the storage tank to be fed with the hydrogen peroxide solution to be purified or to allow the storage tank to be drained of the purified hydrogen peroxide solution. In this way, it is possible to avoid stoppages in the unit and therefore a reduction in production capacity.
A control system makes it possible to obtain a constant flow rate throughout the system. The specific flow rate is between 1 hxe2x88x921 and 10 hxe2x88x921 and more preferably between 10 and 20 hxe2x88x921.
The process is characterized in that it preferably operates continuously. When the system has to be shut down, for example for the operator to carry out maintenance operations, it is preferably rinsed with ultrapure water. The resistivity of the water used will preferably be greater than or equal to 18 Mxcexa9.cm at 25xc2x0 C.
Preferably, in order to avoid any risk of decomposition of the hydrogen peroxide in contact with the resins, the process of the invention is carried out at a temperature of between 0 and 25xc2x0 C., preferably between 0 and 15xc2x0 C. and even more preferably at a temperature substantially equal to about 5xc2x0 C.
In order to avoid problems of resin degradation, the treated aqueous solutions are rarely aqueous hydrogen peroxide solutions containing more than 60% by weight of hydrogen peroxide. The solutions treated according to the invention are generally aqueous solutions which contain from 10 to 60% by weight of hydrogen peroxide. The process is most particularly suitable for the purification of aqueous solutions containing 30% by weight, especially those intended to be used in the electronics industry.
According to another aspect of the invention, a safety system is used which allows the resin bed to be rapidly flooded under a deluge of water, as mentioned above.
The safety system makes it possible, automatically or manually, for the resins to be very rapidly rinsed with water. Preferably, the resins will be rinsed when the flow of liquid is stopped or when the flow rate is less than 50% of the set flow rate because of a malfunction and when the hydrogen peroxide is liable to remain in contact with one of the resins. The device for making the unit safe consists of separate lines of larger diameter than the lines used for the hydrogen peroxide purification. Advantageously, the rinsing water will flow upwards through the columns.
Preferably, the deluging will be effected with very pure water available on the site so as not to introduce impurities and so as to be able to restart the system very rapidly after having repaired the anomaly. However, if ultrapure water is not available, the system will be rinsed with mains town water so as to make the plant safe. A device consisting of valves and sensors is used to switch from ultrapure water to mains town water.
In one version of the process, the town water may be replaced with a store of water, such as a pressurized container for example. Preferably, the volume of this container will be at least 10 times the total volume of the columns.
The unit preferably comprises pneumatic pumps supplied with compressed air. Should there be a break in the compressed-air supply, the pumps are then supplied with nitrogen, optionally xe2x80x9celectronicxe2x80x9d-grade nitrogen, until the compressed air is available again and for at most a time t set beforehand in the system for automatically managing the unit. After this time t, the unit will be rinsed with the deluge of water, as explained above.
The present invention also relates to a unit and an apparatus for the purification of an aqueous hydrogen peroxide solution containing organic and/or metallic impurities.
This apparatus preferably comprises:
at least one column containing an anionic resin;
at least one column containing a cationic resin;
a tank containing the liquid, especially hydrogen peroxide, to be purified in contact with the resins;
a pump, preferably a pneumatic pump, which makes it possible to send the liquid and the hydrogen peroxide into the bottom of the first column;
preferably, a buffer tank and a pump which are placed after each column, the liquid level in each of these tanks being regulated.
The column containing the anionic resin is preferably placed before the column containing the cationic resin, the columns preferably being fed with an up flow.
Each of the columns of the said apparatus generally comprises:
means which allow the resin balls to be kept compacted;
means, placed at the bottom of the column, which allow good distribution of the liquid in the resin bed (for example, a funnel-shaped injection system);
a line, at the bottom of the column, which allows the hydrogen peroxide solution or the rinsing water to flow;
a line, at the top of the column, which allows the hydrogen peroxide solution or the rinsing water to flow out by overflow into a buffer tank.